Korea University study proposes first toxicity-based exposure limits proposed for indoor airborne fungi

Microbial contaminants like airborne bacteria and fungi can make up more than a third of indoor air pollution. Yet, while many countries regulate chemical pollutants, most have no legally binding thresholds for microbial exposure, particularly fungi. This lack of toxicological benchmarks leaves a major gap in indoor air safety policy.

Addressing this gap, a new study led by Professor Wonsuck Yoon of Korea University provides the first species-specific health risk estimates for indoor airborne microbes based on animal toxicity data. This study was made available online on June 9, 2025, and was published on Sep 5, 2025, in Volume 495 of the Journal of Hazardous Materials. This is the first study to estimate human health risks from indoor microbes using benchmark doses derived from animal toxicity data.

Prof. Yoon and his team collected microbial samples from more than 500 high-traffic indoor environments across South Korea, including restaurants, retail stores, and public transit hubs, and identified the most prevalent fungi and bacteria. Four representative species were then selected for controlled exposure studies in mice. Over four weeks, the animals were repeatedly administered doses of these inactivated microbes via their airways.

The results showed clear toxic effects: elevated levels of inflammatory cytokines, increased immune cell infiltration, and lung tissue damage, more specifically from fungal species. Based on these findings, the team calculated species-specific benchmark dose values and converted them into estimated safe exposure limits for humans.

“This work is a part of a national initiative on the health impact of indoor biological agents and highlights the limitation of total microbial load regulation, emphasizing that health risks can vary significantly by species,” said Prof. Yoon.

The study determined that exposure to fungi may be unsafe even at levels below current South Korean and World Health Organization guidelines. By contrast, the estimated safe limits for bacteria were consistent with existing regulations. These findings could inform multiple applications: regulatory policy, building health certification, performance standards for air purifiers, and occupational safety in high-density spaces.

The researchers emphasize the longer-term importance of moving beyond crude microbial counts. “This approach will enable safer, smarter, and better health-optimized indoor environments across diverse urban and residential settings,” said Prof. Yoon.

While the study offers a valuable first step toward species-specific microbial standards, the authors note that it was based on inactivated microbes and limited to culturable strains. Future work should include viable organisms and more comprehensive indoor microbiome assessments to better reflect real-world exposure and risks.

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Reference                                    

Title of original paper: Assessment of pulmonary toxicity in mice to evaluate human health risks from indoor airborne microorganisms

Journal: Journal of Hazardous Materials

DOI: 10.1016/j.jhazmat.2025.138884

About Korea University College of Medicine

Korea University College of Medicine is the medical school of Korea University. It is located in Seoul, South Korea. As one of the oldest medical schools in South Korea, it has been historically regarded as one of the country's top medical schools. The school was founded as Chosun Women's Medical Training Institute in 1928 by Rosetta Sherwood Hall. The institute was subsequently renamed several times and ultimately merged with Korea University to become Korea University College of Medicine. So far, the school has produced over 7,000 graduates, most of whom are working as prominent physicians and public health advocates worldwide.

Website: https://medicine.korea.ac.kr/en/index.do

About the author

Prof. Wonsuck Yoon is the lead author and director of the Air Microbiome Health Research Institute and a professor at Korea University College of Medicine, affiliated with the Allergy and Immunology Research Center. He serves as the principal investigator of the Korean Ministry of Environment’s Indoor Air Biological Hazards Health Impact Assessment Program. His research integrates microbiome science with environmental toxicology, focusing on the health effects of airborne microbes. He also serves as the only board member from Asia in the EAACI (European Academy of Allergy and Clinical Immunology) Working Group on One Health.